In general, a fuel cell is such that, inversely to electrolysis of water, hydrogen is coupled with oxygen and electricity and heat generated thereupon are taken out. Because of their higher electricity generation efficiency and adaptability to environment, fuel cells have been actively developed for household-fuel-cell cogeneration systems and fuel-cell-powered automobiles. Hydrogen as fuel for such fuel cells is produced by reforming, for example, petroleum fuel such as naphtha or kerosene or city gas through a reformer.
FIG. 1 shows a whole system for a residential type polymer electrolyte fuel cell (PEFC) as an example of an installation with a reformer in which reference numeral 1 denotes a reformer; 2, a water vaporizer to vaporize water into water vapor through heat of exhaust gas from the reformer 1; 3, a primary fuel gasifier to gasify primary fuel such as naphtha through heat of the exhaust gas; 4, a desulfurizer to desulfurize source gas to be fed to the reformer 1; 5, a low-temperature shift converter to lower the reformed gas from the reformer 1 to a required temperature (about 200-250° C. or so) through cooling water so as to change CO and H2O into CO2 and H2; 6, a selective oxidation CO remover which removes CO by an oxidation reaction from reformed gas passed through the shift converter 5 controlled by cooling water; 7, a humidifier to humidify the reformed gas having passed through the CO remover 6; and 8, a PEFC with a cathode 8a and an anode 8b. 
In the installation shown in FIG. 1, water is vaporized by the vaporizer 2 into water vapor while the fuel such as naphtha is gasified by the gasifier 3 into source gas. The source gas mixed with the water vapor is guided to the desulfurizer 4 and the source gas desulfurized in the desulfurizer 4 is guided to the reformer 1. The gas reformed by the reformer 1 is guided via the shift converter 5, CO remover 6 and humidifier 7 to the anode 8b of the PEFC 8 while the air is guided through the humidifier 7 to the cathode 8a of the PEFC 8, thereby generating electric power. Anode off-gas from the anode 8b is re-utilized as fuel gas in the reformer 1 while the water from the cathode 8a is utilized as cooling water for the PEFC 8, CO remover 6 and shift converter 5 and as part of the water vapor to be mixed with the source gas.
Conventionally, the reformer 1 and its associated instruments or the vaporizer 2, gasifier 3, desulfurizer 4, shift converter 5 and CO remover 6 are assembled as an unit into a fuel reforming apparatus as shown in FIG. 2. The reformer 1 in the fuel reforming apparatus comprises a reforming vessel body 9 to which the anode off-gas is fed as fuel gas and to which the air is introduced. In the vessel body 9, a first catalytic combustor 10 is arranged to burn the fuel gas to attain rise in temperature. In the vessel body 9 and downstream of the combustor 10, a reforming cylinder 12 is arranged coaxially of the vessel body 9, the cylinder 12 being charged with reforming catalysts (not shown) and having the source gas passed therethrough for reforming of the gas. In the vessel body 9 and peripherally on the cylinder 12, a second catalytic combustor 11 is arranged to burn again the combustion exhaust gas, which has been lowered in temperature due to heat exchange with the source gas flowing through the cylinder 12, so as to attain rise in temperature.
The anode off-gas fed as fuel gas to the reformer 1 is low in calorie and is hard to ignite. However, in the reformer 1 of the fuel reforming apparatus as shown in FIG. 2, exothermic heat is generated through an oxidation reaction forcibly conducted in the combustor 10 as the fuel gas and air are fed to the vessel body 9. Using this as heat source, reforming is conducted as the source gas passes through the reforming catalysts (not shown) in the cylinder 12, thereby generating reformed gas which passes through the shift converter 5 and CO remover 6 to be discharged as CO-free reformed gas. The combustion exhaust gas lowered in temperature by removal of heat due to the reforming reaction of the source gas is burnt again for temperature rising in the combustor 11, is heat-exchanged with the water for reformation in the vaporizer 2 and with the primary fuel such as naphtha in the gasifier 3 and then is discharged.
With the reformer 1 in the fuel reforming apparatus as shown in FIG. 2, however, the combustion temperature cannot be made higher than a tolerable temperature of the catalysts in the combustor 10, so that air must be oversupplied for dilution, resulting in increased air/fuel ratio. As a result, heat transfer area must be increased, leading to increase in length of the reformer 1.
Since the fuel reforming apparatus itself is desired not to have substantial height, the CO remover 6, desulfurizer 4, shift converter 5 and the like are arranged around the reformer 1 as shown in FIG. 2. The reformer 1 is a device to conduct heat exchange at very high temperature (about 750° C. or so) and reaction temperature level therein is different from that in the shift converter 5 or in the CO remover 6, so that heat insulating material 13 such as ceramic fiber is charged between the respective instruments and the whole is covered with and enclosed by heat insulating material 13 to provide a heat insulating layer 14.
However, charging the heat insulating material 13 such as ceramic fiber between the respective instruments and enclosing the whole by the material 13 to provide the heat insulating layer 14, as mentioned above, will increase in volume the layer 14 over the fuel reforming apparatus and construction work for them will be laborious and time-consuming. Moreover, whenever the catalysts in the reformer 1 are to be exchanged and/or maintenance such as inspection is to be conducted, the heat insulating material 13 must be removed and the reforming vessel body 9 must be cut, which will be disadvantageously much laborious and time-consuming.
When the conventional fuel reforming apparatus as shown in FIG. 2 having been lowered in temperature to a room temperature is to be started, startup fuel is burnt in the catalytic combustors 10 and 11 as heat source, which may heat the cylinder 12, vaporizer 2 and gasifier 3 through heat exchange with the combustion gas generated, but does not heat the shift converter 5 and CO remover 6 which are kept warm through heat insulation to outside and cannot be heated from outside. They may be raised in temperature only by flowing of nitrogen and/or steam therethrough; however, nitrogen and steam are generally of not so high temperature. Therefore, it takes much time to heat the reactors such as the shift converter 5 and CO remover 6 to required temperatures.
The invention was made in view of the above and has its object to provide a fuel reforming apparatus which can make its unnecessary to charge heat insulating material such as ceramic fiber between instruments, which enables decrease in volume of the heat insulating layer, which can bring about compactness in size of the apparatus and increased thermal efficiency, which can drastically relieve time and labor for construction work of the heat insulating layer and which can facilitate maintenance. The invention also provides a method for starting a fuel reforming apparatus which can shorten startup time.